The Focus is the conference state server. It is implemented as a SIP user agent that is addressable by using a conference URI. The Focus runs in the User Services module of all Front End Servers. All group IM, multiparty A/V, and data collaboration sessions are managed on the server by the Focus.

The Focus is responsible for the following tasks:

• Initiating conferences
  
  
• Enlisting required conferencing servers
  
  
• Authenticating participants before allowing them to enter a conference
  
  
• Enforcing the policy that specifies whether the meeting organizer is authorized to invite external users
  
  
• Maintaining SIP signaling relationships between conference participants and conferencing servers
  
  
• Managing conference state
  
  
• Accepting subscriptions to conferences and notifying users of changes in conference state, such as the arrival and departure of participants and the addition or removal of media
  
  
• Maintaining and enforcing conference policies and rosters
  
  

The Focus also enables the presenter to lock a meeting so that no more participants can enter.

A separate instance of the Focus exists for each active conference.

The Focus Factory is responsible solely for scheduling meetings. When a user creates a new meeting, the meeting client sends a SIP SERVICE message to the Focus Factory, which creates a new instance of the meeting in the conference database and returns information about the newly created meeting to the client.

Supporting multiparty conferences requires a server known as a conferencing server (also known as an MCU or multipoint control unit). A conferencing server is a pluggable component that is responsible for managing one or more media types. Office Communications Server 2007 R2 includes five conferencing servers:

• IM Conferencing Server.Enables group IM by relaying IM traffic among all participants. When a third participant is added to a two-party IM conversation, the initiating client invites the IM Conferencing Server to the conversation. From that point, all messages among the participants are routed through the IM Conferencing Server.
  
  
• Telephony Conferencing Server. Manages audio conferencing provider (ACP) integration. Supports both dial-out and dial-in, as well as standard third-party call control features such as mute and eject. The Telephony Conferencing Server does not support mixing VoIP and PSTN in the same call. To connect dial-out to PSTN endpoints, a Mediation Server is required; for details, see VoIP Components.
  
  Office Communications Server 2007 R2 adds a new Dial-in Conferencing feature, which provides an on-premises audio bridge that external users can use to dial in to the conference with PSTN devices. For details, see New Dial-in Conferencing Featurein New Server Features in the Getting Started documentation and Dial-in Conferencing Architecture.
  
  
• Web Conferencing Server.Manages conference data collaboration, including native support for Microsoft Office PowerPoint presentations, Microsoft Office document sharing, white boarding, application sharing, polling, Q&A, compliance logging, annotations, meeting summaries, handouts, and various multimedia formats. The Web Conferencing Server uses Persistent Shared Object Model (PSOM), a Microsoft Office Live Meeting protocol, for uploading slides to a meeting.
  
  
• A/V Conferencing Server.Provides multiparty IP audio and video mixing and relaying, including the Microsoft RoundTable conferencing device, by using Secure Real-time Transport Protocol (SRTP) and Secure Real-time Transport Control Protocol (SRTCP).
  
  
• Application Sharing Server.New for Office Communications Server 2007 R2, this conferencing server enables the Desktop Sharing feature, which enables users to transmit a view of their desktop to others during a conference. For details about this feature, see New Desktop Sharing Featurein New Server Features in the Getting Started documentation.
  
  

When the Focus requests a particular conferencing server for a meeting, the Focus sends the request to Conferencing Server Factory, which determines which conferencing server is available to service the request and returns its URL to the Focus. The Conferencing Server Factory is responsible for provisioning a meeting for a particular media type on a Conferencing Server by using the local policies for creating meetings. A Conferencing Server Factory provisions meetings according to local policies and takes into account the current load on the Conferencing Servers before assigning one to a meeting.

Office Communications Server requires the IIS component to be installed on every Front End Server. Office Communications Server relies on IIS for the following functions:

• The Live Meeting client uses IIS to download meeting content (such as PowerPoint presentations).
  
  
• Office Communicator uses IIS to download Address Book Server files.
  
  
• Address Book Web Query service uses IIS to enable Microsoft Office Communicator Mobile for Windows Mobile clients to query for global address list information without downloading Address Book files.
  
  
• Device Update Service uses IIS to provide updated versions of client software, such as Microsoft Office Communicator and Office Communicator Phone Edition, to users.
  
  
• An ASP.NET application running on top of IIS is used for the Group Expansion Web Service, which enables Office Communicator to expand distribution groups for purposes of group IM.
  
  

Other important Front End Server components include the following:

• SIP Proxy.The SIP Proxy (also known as the protocol stack or SIP stack) is the core protocol platform on which all other services are built. It provides the basic structure for networking and security and performs connection management, message header parsing, routing, authentication, and state management.
  
  
• HTTP.SYS.The IIS kernel-mode HTTP protocol stack. HTTP.SYS queues and parses incoming HTTP requests and caches and returns application and site content.
  
  
• User Services.Provides closely integrated IM, presence, and conferencing features built on top of the SIP Proxy. It also includes the Focus and Focus Factory.
  
  
• User Replicator.Updates the user database to be synchronized with Active Directory Domain Services (AD DS). The Address Book Server uses information provided by User Replicator to update information from the global address list.
  
  
• Server API.Provides basic scripting capability for creating custom message filters and routing applications. The scripts can either run in process or, where required, can be dispatched to a managed code application that is running in a separate process.
  
  
• RTC Aggregate Application.Handles the aggregation of presence information across multiple endpoints.
  
  
• Address Book Server.Provides global address list information from Active Directory Domain Services to Office Communicator clients. It also retrieves user and contact information from the RTC database, writes the information to the Address Book files, and then stores the files on a shared folder where they are downloaded by Office Communicator clients. The Address Book Server writes the information to the RTCAb database, which is used by the Address Book Web Query service to respond to user search queries from the 2007 R2 version of Office Communicator Mobile. It optionally normalizes enterprise user phone numbers that are written to the RTC database for the purpose of provisioning user contacts in Office Communicator. The Address Book Server is installed by default on all Front End Servers. The Address Book Web Query service is installed by default on all Web Components Servers.
  
  
• Intelligent IM Filter.Filters incoming IM traffic by using administrator-specified criteria. It is used to block unsolicited or potentially harmful IM items from unknown endpoints outside the corporate firewall.
  
  
• VoIP Components.For details about Front End Server components used for VoIP, see VoIP Components.
  
  
• Archiving Agent.Captures IM for archiving and sends it to a destination queue on an Archiving Server.
  
  
• CDR Agent.Intercepts SIP messages and uses them to send Call Detail Record (CDR) data to a destination queue on a Monitoring Server.
  
  
• QoE Agent.Receives SIP service messages that report call quality metrics and sends them to a destination queue on a Monitoring Server.